Griffin is a finite element-based reactor multiphysics application. It is suitable for steady state and time-dependent coupled neutronics calculations leveraging the various MOOSE-based thermal-fluids applications (Pronghorn, RELAP-7, SAM, Sockeye, etc.) and the fuel performance application (BISON). Griffin solves the linearized Boltzmann transport equation in 1D, 2D, and 3D heterogeneous and homogeneous geometries. It has been used in the analysis of pebble bed reactors (PBRs, PB-FHRs), prismatic reactors (PMRs), molten-salt reactors (MSRs), fast sodium-cooled reactors (FSRs), microreactors, nuclear thermal propulsion (NTPs), and several experimental facilities.